The coextrusion of multilayer sheets and other articles is known in the art. Coextrusion of polymeric webs can occur by passing different melt streams from different extruders into a multiple layer feed block and a film die, or into a multiple manifold die. The feedblock technique merges at least two different materials in a feedblock and then feeds them to a die as a layered stack, which becomes a layered sheet as it leaves the die. A multiple manifold die, on the other hand, combines different molten streams from different extruders at a die lip.
Coextrusion of polymers has been particularly used to form polymeric webs with discontinous layers of at least one polymeric material within a matrix of another polymeric material. Examples of some uses for such multilayer articles include tape backings and medical articles. One problem associated with these coextruded articles is that the known apparatuses used to form such articles are designed such that the articles produced have weld lines formed in the y-z plane of the articles.
A weld line in an article is the meeting of two flows of molten polymers when the article is made. The two flow fronts do not bond well and give a weakened inter-flow-front surface bonding. Weld lines can result where two different polymer flows come together. In addition, they can be formed where one polymer flow has been divided and then rejoined. The morphology and the mechanical properties of articles with weld lines differ substantially from those that are devoid of them. In general, the weld lines exhibit an orientation of the material which is parallel to the plane of the weld line, and give rise to considerable embrittlement of the articles.
In particular for multilayer thin film articles, it is desirable that the articles be substantially free of weld lines in the y-z plane. It is inevitable that weld lines will be formed, in coextruded multilayer articles, in the x-y plane, because weld lines will be formed between the different layers in the multilayer webs. However, it is weld lines that are in the y-z plane that are particularly not desired.
Thus a need exists for polymeric coextruded multilayer articles, and a method of making the same, having more than one layer comprising a plurality of distinct phases that are discontinuous in the cross-web direction, and at least one layer that is substantially continuous in both the cross-web and down-web directions, and wherein the substantially continuous layer or layers of the article lack weld lines in the y-z plane.